Self-reinforced silicon nitride ceramic of high fracture toughness

ABSTRACT

A process for preparing a self-reinforced silicon nitride ceramic body of high fracture toughness comprising hot-pressing a powder mixture containing silicon nitride, magnesium oxide, yttrium oxide and calcium oxide under conditions such that densification and the in situ formation of β-silicon nitride whiskers having a high aspect ratio occur. A novel silicon nitride ceramic of high fracture toughness is disclosed comprising a β-silicon nitride crystalline phase wherein at least about 20 volume percent of the phase is in the form of whiskers having an average aspect ratio of at least about 2.5; a glassy second phase containing magnesium oxide, yttrium oxide, calcium oxide, and silica in a total amount not greater than about 35 weight percent; and not greater than about 10 weight percent of the total weight as other phases.

This is a divisional, of application Ser. No. 07/148,748, filed January27, 1988 now U.S. Pat. No. 4,883,776.

BACKGROUND OF THE INVENTION

This invention pertains to a silicon nitride (Si₃ N₄) ceramic body and aprocess for preparing the ceramic body.

Silicon nitride ceramics are recognized for their excellent mechanicaland physical properties, including good wear resistance, low coefficientof thermal expansion, good thermal shock resistance, high creepresistance and high electrical resistivity. In addition, silicon nitrideceramics are resistant to chemical attack, particularly to oxidation.For these attributes silicon nitride is useful in a variety of wear andhigh temperature applications, such as cutting tools and parts in pumpsand engines.

Failure of silicon nitride ceramics is generally associated withbrittleness and flaws. The object therefore is to prepare a siliconnitride ceramic with high fracture toughness (K_(IC)) and strength.Fracture strength is directly proportional to the fracture toughness andinversely proportional to the square root of the flaw size. Highfracture toughness combined with small flaw size is therefore highlydesirable. Monolithic silicon nitride, however, has a relatively lowfracture toughness of about 5 MPa (m)^(1/2).

U.S. Pat. No. 4,543,345 teaches that the addition of silicon carbidewhiskers to ceramic materials can result in an increase in the fracturetoughness. Silicon carbide whiskers have a single crystal structure andare in a size range of about 0.6 μm in diameter and about 10 μm to about80 μm in length. This technique, however, does not provide significanttoughening in the case of silicon nitride ceramics. Moreover, the use ofsilicon carbide whiskers is associated with serious processing problems.The whiskers have a tendency to agglomerate and settle. It is difficultto deagglomerate the whiskers without significantly destroying thewhiskers' length. In addition, the whiskers are difficult tomanufacture; thus, they display inconsistent properties and are costly.It would be highly desirable to have a silicon nitride ceramic of highfracture toughness which does not require the presence of siliconcarbide whiskers.

It is known that the high temperature strength of hot-pressed siliconnitride ceramics can be increased by crystallization of thegrain-boundary glass phase (second phase). This has been demonstrated ina hot-pressed composite containing beta(β)-silicon nitride and acrystalline second phase of Si₃ N₄.Y₂ O₃, as reported by Akihiko Tsugeet al. in the Journal of the American Ceramics Society, 58, 323-326(1975). However, the fracture toughness of this silicon nitride is only5-6 MPa (m)^(1/2).

It is also known that the presence of β-silicon nitride with a highaspect ratio can increase the fracture toughness of silicon nitrideceramics, as reported by F. F. Lange, in the Journal of the AmericanCeramics Society, 62 (12), 1369-1374, (1983). "Aspect ratio" is definedas the ratio of the length of the whisker to the diameter or width ofthe whisker. Thus, whiskers with a high aspect ratio are fibrous innature. If such whiskers are also strong, crack propagation must take atortuous path around the whiskers, thereby leading to high fracturetoughness. The transformation of alpha(α)-silicon nitride to β-siliconnitride takes place above 1600° C.; however, crystals of the beta phaseprecipitate usually as a mixture of equiaxed grains and elongated grainswith a low aspect ratio. Reproducible control of the aspect ratio is adifficult problem.

Typically, the prior art is silent with regard to aspect ratio andfracture toughness of silicon nitride ceramics. U.S. Pat. No. 4,279,657,for example, discloses a powder dispersion containing silicon nitrideand magnesium oxide which is hot-pressed to form a light-transmittingsilicon nitride ceramic. The ceramic is disclosed to comprise more than50 weight percent β-silicon nitride ranging in grain size from 1 μm toabout 10 μm, but typically less than 5 μm. This patent teaches thatimpurities, such as calcium, in a total amount greater than about 0.1weight percent are undesirable. U.S. Pat. No. 4,227,842 discloses acutting tool consisting essentially of beta phase silicon nitride andyttrium oxide. The tool is prepared by hot-pressing a powder mixture ofα-silicon nitride and yttrium oxide to obtain a ceramic of 100 percenttheoretical density. U.S. Pat. No. 4,652,276 teaches a cutting toolcomprising a granular phase consisting essentially of β-silicon nitrideand an intergranular amorphous phase consisting essentially of magnesiumoxide from about 0.5 to about 10 weight percent, yttrium oxide fromabout 2.5 to about 10 weight percent, silicon oxide in an amount lessthan about 2.5 weight percent, and the balance less than 5 weightpercent impurities such as aluminum. The tool is prepared byhot-pressing.

It would be very desirable to have a silicon nitride ceramic of highfracture toughness and high fracture strength. It would be advantageousif such a strong silicon nitride ceramic could be prepared withoutsilicon carbide reinforcing whiskers. Moreover, it would be highlydesirable to have a process which would be reproducible, inexpensive,and amenable to industrial scale-up for preparing such a tough andstrong silicon nitride ceramic.

SUMMARY OF THE INVENTION

In one aspect this invention is a process for preparing aself-reinforced silicon nitride ceramic body containing predominatelyβ-silicon nitride whiskers having a high average aspect ratio. Theprocess comprises preparing a powder mixture comprising:

(a) silicon nitride in an amount sufficient to provide a ceramic body;

(b) magnesium oxide in an amount sufficient to promote densification ofthe powder;

(c) yttrium oxide in an amount sufficient to promote the essentiallycomplete conversion of the starting silicon nitride to β-siliconnitride; and

(d) calcium oxide in an amount sufficient to promote the formation ofβ-silicon nitride whiskers,

and hot-pressing the powder under conditions such that densification andin situ formation of β-silicon nitride whiskers having a high averageaspect ratio occur. In this manner a self-reinforced silicon nitrideceramic body having a fracture toughness greater than about 6 MPa(m)^(1/2), as measured by the Chevron notch technique describedhereinbelow, is formed. For the purposes of the present invention a"high" average aspect ratio means an average aspect ratio of at leastabout 2.5.

In another aspect this invention is a silicon nitride ceramic bodyhaving a fracture toughness greater than about 6 MPa (m)^(1/2), asmeasured by the Chevron notch technique described hereinbelow,comprising:

(a) a crystalline phase of β-silicon nitride of which at least about 20volume percent, as measured in a plane by scanning electron microscopy,is in the form of whiskers having an average aspect ratio of at leastabout 2.5; and

(b) a glassy phase in an amount not greater than about 35 weight percentof the total weight and comprising magnesium oxide, yttrium oxide,calcium oxide, and silica.

In a third aspect this invention is a cutting tool comprising theabove-identified silicon nitride ceramic body.

Unexpectedly, the silicon nitride ceramic body of this inventionexhibits a significantly higher fracture toughness than the monolithicor whisker-reinforced silicon nitride ceramics of the prior art.Moreover, if the fracture toughness of the silicon nitride ceramic ofthis invention is normalized with respect to density, the normalizedtoughness is among the highest known for any ceramic material.Advantageously, the silicon nitride ceramic of this invention isself-reinforced and does not require the presence of expensive siliconcarbide whiskers. More advantageously, the process for preparing thenovel, self-reinforced silicon nitride ceramic body of this invention isreproducible, amenable to industrial scale-up, and less expensive thanprocesses using silicon carbide whisker reinforcement.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a photomicrograph of the silicon nitride ceramic compositionof this invention. This photomicrograph is representative of thepreferred embodiments of this invention. Elongated whiskers of β-siliconnitride are readily observed.

DETAILED DESCRIPTION OF THE INVENTION

The silicon nitride starting material which is used in the preparationof the ceramic body of this invention can be any silicon nitride powder,including the crystalline forms of α-silicon nitride and β-siliconnitride, or noncrystalline amorphous silicon nitride, or mixturesthereof. Preferably, the silicon nitride powder is predominately in thealpha crystalline form or the amorphous form, or mixtures thereof. Morepreferably, the starting silicon nitride is predominately in the alphacrystalline form. It is also advantageous if the preferred startingpowder possesses a high α/β weight ratio. Preferably, the startingpowder contains no greater than about 20 weight percent β-siliconnitride; more preferably, no greater than about 10 weight percentβ-silicon nitride; most preferably, no greater than about 6 weightpercent β-silicon nitride.

Generally, the higher the purity of the starting silicon nitride powder,the better will be the properties of the finished ceramic body.Depending on the source, however, the silicon nitride powder may containmetallic and nonmetallic impurities. Some impurities may be tolerated inthe powder, although it is preferred to minimize these as much aspossible. Oxygen, for example, is present to some extent in the form ofsilica, SiO₂, which usually is found as a coating on the surface of thesilicon nitride particles. Preferably, the oxygen content is no greaterthan about 5 weight percent. Above this preferred upper limit, thesilica may create a large glassy phase which may lower the hightemperature properties of the finished ceramic. More preferably, theoxygen content is no greater than about 2.5 weight percent; mostpreferably, no greater than about 2.0 weight percent. In addition tooxygen, elemental silicon is usually present in amounts ranging up toabout 0.5 weight percent. These amounts of elemental silicon are notdeleterious and can be tolerated. Other nonmetals, such as carbon whichis likely to form silicon carbide during hot-pressing or sintering, aretolerable in small amounts. In addition to nonmetallic contaminants, thesilicon nitride powder can be contaminated with metals, such as iron,aluminum, and lead. These may create low-melting intergranular phaseswhich lower high temperature properties in the finished ceramic. Ingeneral, the total percentage of metallic contaminants preferably shouldnot exceed about 0.5 weight percent; and more preferably should notexceed about 0.1 weight percent. Iron is particularly deleteriousbecause it forms a brittle iron silicide, which can reduce the strengthof the Si₃ N₄ ceramic. It is preferred, therefore, that the siliconnitride starting powder contains less than about 1000 ppm iron. Morepreferably, the silicon nitride powder contains less than about 700 ppmiron; even more preferably, less than about 500 ppm iron; mostpreferably, less than about 250 ppm iron. Unexpectedly, as describedhereinbelow, calcium has been found to be advantageous; therefore,silicon nitride powders doped with up to about 5.3 weight percentcalcium oxide are desirable. Such high levels of calcium oxide are nottypically available in commercial silicon nitride powders, and it ismore common to find powders containing only 100 ppm or less of calciumoxide.

The silicon nitride starting powder can be of any size or surface areaprovided that the ceramic body of this invention is obtained byhot-pressing. Large particles having an average diameter in the rangefrom about 15 μm to about 50 μm, for example, may be in the form of hardagglomerates which cannot be easily broken. Powders containing suchagglomerates make poor ceramics. On the other hand, very fine powdershaving an average diameter less than about 0.2 μm are difficult toobtain uniformly and to process. Preferably, the particles have anaverage diameter in the range from about 0.2 μm to about 10.0 μm; morepreferably, from about 0.5 μm to about 3.0 μm. Preferably, the surfacearea of the silicon nitride particles is in the range from about 5 m² /gto about 15 m² /g, as determined by the Brunauer-Emmett-Teller (BET)method of measuring surface area, which is described by C. N.Satterfield in Heterogeneous Catalysis in Practice, McGraw-Hill BookCompany, 1980, pp. 102-105. More preferably, the surface area is in therange from about 8 m² /g to about 15 m² /g.

In the process of this invention it is required to mix the startingsilicon nitride powder, described hereinabove, with a combination ofmetal oxides to obtain a powder mixture, which is used in preparing thetough silicon nitride ceramic body of this invention. These metal oxidesare magnesium oxide (MgO), yttrium oxide (Y₂ O₃), and calcium oxide(CaO). Ordinarily, the total quantity of metal oxides is no greater thanabout 35 weight percent of the total weight. The total quantity of metaloxides will depend, however, on the end use application of the firedceramic. Preferably, the total quantity of metal oxides is in the rangefrom about 15 weight percent to about 35 weight percent for mediumtemperature and/or the highest fracture toughness applications. By"medium temperature" it is meant temperatures in the range from about900° C. to about 1200° C. Ceramic cutting tools are an example of amedium temperature and very high fracture toughness application.Preferably, the total quantity of metal oxides is in the range fromabout 3 weight percent to about 15 weight percent for high temperatureand/or moderately high fracture toughness applications. By "hightemperature" it is meant temperatures from about 1200° C. to about 1400°C. Parts for ceramic engines are an example of a high temperature andmoderately high fracture toughness application.

Raw silicon nitride powders cannot be densified to high densities in theabsence of densification aids, such as refractory oxides, or nitrides,or a combination of these. Thus, magnesium oxide is admixed with thesilicon nitride starting powder in a manner described hereinbelow forthe purpose of promoting densification of the silicon nitride duringprocessing. Ordinarily, the magnesium oxide forms a liquid phase atbetween about 1300° C. and 1500° C. into which the α-silicon nitridedissolves. The rate of mass transport of the α-silicon nitride is fastin magnesium oxide; thus, the silicon nitride density increases until acritical mass is reached and precipitation occurs. Any amount ofmagnesium oxide which promotes this densification and produces the toughsilicon nitride ceramic body of the invention is acceptable. Preferably,the quantity of magnesium oxide is in the range from about 1.0 weightpercent to about 20.6 weight percent based on the total weight of thepowder mixture. More preferably, the quantity of magnesium oxide is inthe range from about 2.4 weight percent to about 9.8 weight percent;most preferably, from about 4.2 weight percent to about 6.1 weightpercent.

In addition to magnesium oxide, the powder mixture is required tocontain yttrium oxide. Yttrium oxide forms a glassy phase through whichmass transport is considerably slower than in magnesium oxide. Thus,α-silicon nitride dissolves in yttrium oxide on heating, but is notreadily densified. Advantageously, however, yttrium oxide promotes therapid, essentially complete conversion of α-silicon nitride to β-siliconnitride. This conversion is most desirable because the β-silicon nitridein the form of elongated whiskers is responsible for the high fracturetoughness and high fracture strength of the silicon nitride ceramic bodyof this invention. Any amount of yttrium oxide can be employed in thestarting powder providing the quantity is sufficient to cause theessentially complete conversion of the starting silicon nitride toβ-silicon nitride, and is sufficient to produce the tough siliconnitride ceramic body of the invention. Preferably, the amount of yttriumoxide employed is in the range from about 0.7 weight percent to about28.3 weight percent based on the total weight of the powder mixture.More preferably, the amount of yttrium oxide employed is in the rangefrom about 2.6 weight percent to about 12.6 weight percent; mostpreferably, from about 4.3 weight percent to about 8.5 weight percent.

Surprisingly, the weight ratio of yttrium oxide to magnesium oxide hasbeen found to affect the fracture toughness of the finished ceramic,providing calcium oxide is also present in the powder mixture. Anyweight ratio of yttrium oxide to magnesium oxide is acceptable providingthe fracture toughness shows an improvement over the prior art fracturetoughness value of 5 MPa (m)^(1/2) for nonreinforced, monolithic siliconnitride. Preferably, the Y₂ O₃ /MgO weight ratio is in the range fromabout 0.7 to about 4.2. More preferably, the Y₂ O₃ /MgO weight ratio isin the range from about 0.7 to about 2.8; most preferably, from about1.0 to about 1.8. In the absence of calcium oxide the Y₂ O₃ /MgO weightratio has no significant effect on the fracture toughness.

The third oxide required to be present in the powder mixture is calciumoxide. This oxide in particular helps to provide a ceramic body ofsuperior fracture toughness and high strength. Just how the calciumoxide contributes to the excellent physical properties which areobserved in the silicon nitride ceramic body of this invention is notcompletely understood. It is possible that the calcium oxide improvesthe viscosity of the glassy phase thereby facilitating the nucleation ofelongated whiskers of β-silicon nitride; the latter being primarilyresponsible for the improved fracture toughness. The aforementionedtherory is presented with the understanding that such a theory is not tobe binding or limiting of the scope of the invention. Any amount ofcalcium oxide in the starting powder is acceptable providing the amountis sufficient to promote the formation of β-silicon nitride whiskers,described hereinbelow, and sufficient to produce the tough siliconnitride ceramic body of this invention. Preferably, the amount ofcalcium oxide employed is in the range from about 0.01 weight percent toabout 5.3 weight percent based on the total weight of the powdermixture. More preferably, the amount of calcium oxide employed is in therange from about 0.1 weight percent to about 1.0 weight percent; mostpreferably, from about 0.2 weight percent to about 0.5 weight percent.

It is desirable to use magnesium oxide, yttrium oxide, and calcium oxidepowders which are pure and sufficiently small in size. Purity is nottypically a problem, because commercially available oxide powdersgenerally contain less than 20 ppm each of assorted impurities. Theselevels of impurities are tolerable. Larger amounts of impurities, as forexample in the 0.5 weight percent range, are not recommended as they maycause a change in the final ceramic composition and properties. Iron, asnoted hereinbefore, is particularly deleterious. A small powder particlesize is favored, because dispersion is enhanced by smaller particles.Preferably, the oxide powders have an average particle size no greaterthan about 5 μm in diameter.

The preparation of the powder mixture containing silicon nitride,magnesium oxide, yttrium oxide, and calcium oxide is accomplished in anysuitable manner. Ball-milling of the components in powder form is oneacceptable manner of preparation. The preferred preparation, however,comprises preparing a finely-divided suspension of silicon nitride andthe metal oxide powders in a carrier medium, and drying the suspensionto obtain the powder mixture. The preparation of the finely-dividedsuspension of silicon nitride and metal oxide powders in a carriermedium requires no particular order of addition of the components. Forexample, it is possible to add the oxides to a colloidal suspension ofsilicon nitride in a carrier medium. Alternatively, silicon nitride canbe added to a colloidal suspension of the metal oxide powders in acarrier medium. Preferably, the latter method is employed, as describedhereinbelow.

The carrier medium may be any inorganic or organic compound which is aliquid at room temperature and atmospheric pressure. Examples ofsuitable carrier media include water; alcohols, such as methanol,ethanol and isopropanol; ketones, such as acetone and methyl ethylketone; aliphatic hydrocarbons, such as pentanes and hexanes; andaromatic hydrocarbons, such as benzene and toluene. Preferably, thecarrier medium is water, an alcohol, or a ketone. More preferably, thecarrier medium is water. The function of the carrier medium is to imparta viscosity suitable for mixing to the solid powders. Any quantity ofcarrier medium which achieves this purpose is sufficient and acceptable.Preferably, a quantity of carrier medium is employed such that thesolids content is in the range from about 20 weight percent to about 50weight percent. More preferably, a quantity of carrier medium isemployed such that the solids content is in the range from about 35weight percent to about 45 weight percent. Below the preferred lowerlimit the viscosity of the solid suspension may be too low and thedeagglomeration mixing may be ineffective. Above the preferred upperlimit the viscosity may be too high, and the deagglomeration mixing maybe difficult.

The metal oxides are added to the carrier medium in any manner whichgives rise to a finely dispersed suspension of metal oxides. Typically,the process is conducted in a large vessel at room temperature (taken as23° C.) under air with vigorous stirring. Any common stirring means issuitable, such as a ball-milling device or an attrition mixer. Anultrasonic vibrator may be used in a supplementary manner to break downsmaller agglomerates. The attrition mixer is preferred. Since yttriumoxide tends to flocculate with magnesium oxide and calcium oxide,preferably the latter two are added to the water, and afterwards theyttrium oxide is added. Lastly, the silicon nitride starting powder isadded.

To aid in the dispersion of yttrium oxide and silicon nitride withmagnesium oxide and calcium oxide, optionally one or more surfactants ordispersants can be added to the suspension. The choice of surfactant(s)or dispersant(s) can vary widely as is well-known in the art. Forexample, in aqueous suspensions a strong base makes a suitablesurfactant by provoking a repulsion between the silicon nitrideparticles, thereby improving its dispersion. Any inorganic or organicbase which is soluble in water is acceptable, including ammonia, alkalimetal hydroxides, alkali metal alkoxides, alkylamines, quaternaryammonium hydroxides, and metal silicates. Preferably, the surfactant isa metal silicate. More preferably, the surfactant is sodium silicate.When metal silicates are employed as surfactants, the surfactant isadded to the aqueous suspension of magnesium oxide and calcium oxideprior to the addition of yttrium oxide and silicon nitride. Any amountof surfactant is acceptable providing the dispersion is improved.Typically, the amount of surfactant is in the range from about 0.01 to1.0 weight percent of the powder mixture. Preferably, the amount issufficient to raise the pH of the aqueous suspension to at least about10. More preferably, the pH is raised to a value in the range from about11.0 to about 11.5. Below the preferred lower pH limit and above thepreferred upper pH limit, the solids tend to flocculate. Theconcentration of the surfactant should be strong enough to raise the pHto the desired level without substantially increasing the volume of thesuspension or lowering the viscosity thereof.

After the colloidal suspension comprising silicon nitride, magnesiumoxide, yttrium oxide, and calcium oxide in a carrier medium is prepared,the suspension is further agitated and mixed. The purpose of thisfurther agitation is to break down any remaining agglomerates and assurea uniform, finely-divided suspension. The agitation is carried out bymechanical means, mentioned hereinbefore, preferably for a time in therange from about 30 minutes to about 16 hours depending on the type ofagitator employed. Additionally, an ultrasonic vibrator can be used toaid in the deagglomeration. After the solid suspension is adequatelydispersed, it is typically passed through a sieve to remove anyremaining large agglomerates greater than about 100 μm in diameter.Finally, the pH of the finely-divided suspension is adjusted to about 10in order to increase flocculation and maintain homogeneity during thedrying process.

The finely-divided suspension is now ready for processing intogreenware. For example, the suspension can be slip-cast by techniqueswell-known in the art for eventual sintering. Alternatively, thesuspension can be dried into a powder and ground for use in hot-pressingprocesses. Drying is accomplished by standard drying means, such as byspray-drying or oven drying under a nitrogen purge. Preferably, dryingis accomplished in an oven under a nitrogen purge. During the dryingprocess free carrier medium is removed. The temperature of the dryingdepends on the boiling point of the carrier medium employed. Typically,the drying process is conducted at a temperature just below the boilingpoint of the carrier medium under atmospheric pressure. Preferably, thecarrier medium is water and the temperature of drying is about 90° C.After drying, the resulting powder is sieved through a screen to obtaina powder having a maximum agglomerate diameter of about 100 μm. Thescreen size is usually less than about 60 mesh; more preferably, lessthan about 80 mesh. The powder which is obtained on sieving is thepowder mixture which is used in the hot-pressing process of thisinvention.

The preferred method of processing the powder mixture is byhot-pressing, which comprises heating the powder under pressure toobtain the densified ceramic body. Any standard hot-pressing equipmentis acceptable, such as a graphite die equipped with a heating means anda hydraulic press. The hot-pressing is conducted under an inertatmosphere, such as nitrogen, to prevent the oxidation and decompositionof silicon nitride at high temperatures. The direction of pressing isuniaxial and perpendicular to the plane of the plates.

Any processing temperature and pressure will suffice providing the novelsilicon nitride ceramic of this invention, described hereinbelow, isobtained. Typically, however, the temperature must be carefullycontrolled, because the elongated β-silicon nitride whiskers are foundto form in a narrow temperature range. Preferably, the temperature ismaintained during pressurizing in the range from about 1750° C. to about1870° C. More preferably, the temperature is maintained in the rangefrom about 1800° C. to about 1850° C. Most preferably, the temperatureis maintained in the range from about 1820° C. to about 1840° C. Belowthe preferred lower temperature limit the formation of elongatedβ-silicon nitride whiskers may be retarded. Above the preferred uppertemperature limit the silicon nitride may decompose, and specialpressure equipment may be required to conduct the densification. In amore preferred treatment, the sample is held at about 1700° C. for atime from about 5 minutes to about 15 minutes, cooled to about 1500° C.and then reheated at a temperature in the range from about 1750° C. toabout 1870° C. for a time in the range from about 20 minutes to about 90minutes. This preferred heating procedure produces a greaterconcentration of elongated β-silicon nitride crystals. It is noted thatthe accurate measurement of high temperatures, such as those quotedhereinabove, is technically difficult. Some variation in the preferredtemperature range may be observed depending on the method employed inmeasuring the temperature. The preferred temperatures of this inventionare measured by use of a tungsten-rhenium thermocouple, obtained fromand calibrated by the Omega Company.

While the pressure during hot-pressing is important, it is not quite ascritical a parameter as temperature. Typically, the pressure should besufficient to cause densification of the green body. Preferably, thepressure is in the range from about 3000 psig to about 6000 psig; morepreferably, from about 4000 psig to about 5500 psig; most preferably,about 4500 psig to about 5200 psig. Below the preferred lower pressurelimit the powder will not be sufficiently densified. Above the preferredupper pressure limit the powder will densify in a shorter time and at alower temperature. Although less rigorous processing conditions seem onthe surface to be desirable, the formation of elongated β-siliconnitride crystals may be inhibited at lower temperatures and shorterpressing times.

The amount of time that the powder mixture is heated under pressureshould be sufficient to bring the powder to essentially completedensification. Generally, ram movement is a good indicator of the extentof densification. As long as the ram continues to move, thedensification is incomplete. When the ram has stopped moving for atleast about 15 minutes, the densification is essentially complete atabout 99 percent or greater of the theoretical value. Thus, the timerequired for hot-pressing is the time during ram movement plus about anadditional 15 to 30 minutes. Preferably, the time is in the range fromabout 15 minutes to about 2 hours; more preferably, from about 30minutes to about 90 minutes; most preferably, about 45 minutes to about75 minutes.

The hot-pressing method of densification, described hereinbefore, allowsfor the formation of silicon nitride ceramic articles which can be usedas cutting tools. A variety of shapes can be made by hot-pressing, onecommon shape being a flat plate. These plates may range in size fromabout 2 inches in length by about 1.5 inches in width by about 0.45 inchin thickness to about 16 inches in length by about 16 inches in width byabout 1.0 inch in thickness. Smaller and larger plates can also befabricated, as determined by the size of the hot-pressing plaques.Cutting tools can be fabricated by slicing and grinding these platesinto a variety of cutting tool shapes.

The silicon nitride ceramic body which is produced by the hot-pressingprocess of this invention is a dense material having no significantporosity. Preferably, densification proceeds to greater than 95 percentof the theoretical value; more preferably, to greater than 97 percent ofthe theoretical value; most preferably, to greater than 99 percent ofthe theoretical value. Moreover, as measured by X-ray diffraction, thesilicon nitride is present in the beta crystalline form, indicatingessentially complete alpha to beta conversion during processing. Quiteunexpectedly, the β-silicon nitride is present predominately as singlecrystal, "needle-like" whiskers, as determined by both scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM). The size ofthe hexagonal β-silicon nitride grains is usually in the range fromabout 3 μm to about 20 μm in length with a mean diameter of from about0.5 μm to about 1.5 μm; preferably from about 10 μm to about 20 μm inlength with a mean diameter from about 0.5 μm to about 1.0 μm.

Since the whiskers are oriented randomly, it is difficult to determineexactly the percentage of silicon nitride which exists as whiskers, asopposed to equiaxed particles. The measurement is made by viewing oneplane of the silicon nitride ceramic in a scanning electron microscope(SEM) and measuring the percentage by volume occupied by whiskers havingan aspect ratio between 2 and 16. It is observed that the whiskers arehomogeneously distributed and randomly oriented throughout the ceramicbody, and that the volume occupied by the whiskers is approximately thesame in all planes. Typically, the percentage of silicon nitridewhiskers having an aspect ratio of between 2 and 16 is at least about 20volume percent as measured in a plane. Preferably, the percentage ofsilicon nitride whiskers having an aspect ratio between 2 and 16 is atleast about 35 volume percent as measured in a plane. Unexpectedly, thewhiskers are found to have a high average aspect ratio. Typically, theaverage aspect ratio of the silicon nitride whiskers is at least about2.5; preferably, at least about 5.5. It is noted that the SEM techniquefor measuring the volume percentage of whiskers is a lower bound. Forexample, a whisker which is perpendicular to the plane may have anapparent aspect ratio of less than 2; whereas the true aspect ratio maybe very much greater than 2.

In addition to the β-silicon nitride phase, the ceramic body of thisinvention contains a glassy second phase, which constitutes no greaterthan about 35 weight percent of the total weight. The glassy secondphase has a bulk chemical composition consisting essentially of fromabout 15 weight percent to about 51 weight percent magnesium oxide, fromabout 36 weight percent to about 63 weight percent yttrium oxide, fromabout 3 weight percent to about 30 weight percent silica, and from about0.1 weight percent to about 15 weight percent calcium oxide, asdetermined by neutron activation analysis; and wherein the yttrium oxideto magnesium oxide weight ratio is in the range from about 0.7 to about4.2.

Small quantities of silicon carbide and two unknown phases are presentin a total amount not exceeding about 10 weight percent. One of theunknown phases possesses a fiber-like, layered and ordered structure.The typical size of the particles of this phase is about 500 Å in widthby about 0.7 μm in length.

The mechanical properties of the self-reinforced silicon nitride ceramicbody are readily measured by use of standard tests. In particular,fracture toughness (K_(IC)) is measured according to the Chevron notchand the Palmqvist methods described hereinafter. Fracture strength(modulus of rupture) is measured according to the Military Standard1942b test. Hardness is measured according to the Vickers indentationtest.

Fracture strength (modulus of rupture) measures the resistance of thematerial to fracture under a steady load. Fracture strength is definedas the maximum unit stress which the material will develop beforefracture occurs. Test bars are prepared by cutting rectangular bars (45mm×4 mm×3 mm) of the silicon nitride ceramic in a plane perpendicular tothe pressing direction. The bars are ground on the surfaces parallel tothe pressing plates using a 500 grid grinding wheel (Military Standard1974). The fracture strength is measured at room temperature using a4-point bend test with 20 mm span and crosshead speed of 0.5 mm/min.Typically, the fracture strength at room temperature is at least about650 MPa. Preferably, the fracture strength at room temperature rangesfrom about 685 MPa to about 925 MPa; more preferably, from about 800 MPato about 925 MPa. High temperature strength is measured using a 3-pointbend test with 20 mm span and crosshead speed of 0.5 mm/min. Typically,at 1000° C. the fracture strength is at least about 550 MPa. Preferably,at 1000° C. the fracture strength ranges from about 580 MPa to about 650MPa. Typically, at 1300° C. the fracture strength is at least about 300MPa. Preferably, at 1300° C. the fracture strength ranges from about 400MPa to about 500 MPa.

Toughness measures the resistance of the material to fracture under adynamic load. More specifically, fracture toughness is defined as themaximum amount of energy which a unit volume of material will absorbwithout fracture. In the present invention two methods are employed tomeasure fracture toughness. The first of these is the Chevron notchtest. Test bars are prepared as described hereinabove, and additionallyscored with a Chevron notch. The test bars are then subjected to a3-point bend test with 40 mm span and crosshead speed of 0.5 mm/min.Typically, the fracture toughness of the silicon nitride ceramic body ofthis invention, as measured at room temperature (taken as 23° C.) by theChevron notch technique, is greater than about 6 MPa (m)^(1/2).Preferably, the room temperature fracture toughness is greater thanabout 7 MPa (m)^(1/2) ; more preferably, greater than about 8 MPa(m)^(1/2). Most preferably, the room temperature fracture toughnessranges from about 9 MPa (m)^(1/2) to about 14 MPa (m)^(1/2). Preferably,at 1000° C. the fracture toughness is greater than about 6 MPa(m)^(1/2). More preferably, at 1000° C. the fracture toughness rangesfrom about 7 MPa (m)^(1/2) to about 12 MPa (m)^(1/2).

In the evaluation of cutting tool materials it is useful to measure thePalmqvist toughness and the Vickers hardness. Both measurements can bemade simultaneously on one test sample, and therefore these tests arevery convenient.

The Vickers hardness test measures the resistance of the ceramicmaterial to indentation. A sample, approximately 1 cm in length by 1 cmin width by 1 cm in height, is placed on a flat surface, and indentedwith a standard Vickers diamond indentor at a crosshead speed of 0.02in/min. The Vickers hardness number is calculated from the applied load,in this case 14 kg, and the cross-sectional area of the indentation.Prior to making the test, the test sample is polished in a specialmanner. First, the sample is cleaned and rough spots are flattened byuse of a 220-grid diamond wheel. Next, a 45-micron diamond wheel is usedto start the polishing. Next, the sample is treated to a series ofpolishings at 30 psi and 200 rpm in the following consecutive manner:three five-minute intervals with 30-micron diamond paste, threefive-minute intervals with 15-micron diamond paste, three five-minuteintervals with 6-micron diamond paste, two five-minute intervals with1-micron diamond paste, and one five-minute interval with 0.25-microndiamond paste. Between each interval the sample is throughly cleansed bywashing with water and sonicating for two minutes. The Vickers hardnessnumber of the silicon nitride ceramic of this invention is at leastabout 1325 kg/mm² at room temperature. Preferably, the Vickers hardnessnumber ranges from about 1340 kg/mm² to about 1470 kg/mm² at roomtemperature; more preferably, from about 1370 kg/mm² to about 1470kg/mm².

The Palmqvist toughness test is an extension of the Vickers test. (SeeS. Palmqvist in Jerndontorets Annalen, 141 (1957), 300, for adescription of the Palmqvist toughness test.) The test sample isprepared and indented as in the Vickers test, but the 14-kg load isadditionally held for 15 seconds. The sample cracks. The measurements ofthe indented diagonals and the crack lengths are made on a Nikon UM2microscope at 1000× magnification. The Palmqvist toughness (W) isdirectly proportional to the applied load (P) and inversely proportionalto the crack length (L). Preferably, the silicon nitride ceramic body ofthis invention exhibits a Palmqvist toughness at room temperature of atleast about 37 kg/mm. Preferably, the silicon nitride ceramic body ofthis invention exhibits a Palmqvist toughness at room temperature in therange from about 37 kg/mm to about 52 kg/mm; more preferably, from about45 kg/mm to about 52 kg/mm.

ILLUSTRATIVE EMBODIMENTS

The following examples serve to illustrate the novel self-reinforcedsilicon nitride composition of this invention, the method of preparingthe novel silicon nitride ceramic, and the utility of the composition asa cutting tool. The examples are not intended to be limiting of thescope of this invention. All percentages are weight percent unlessotherwise noted.

EXAMPLE 1

Materials: Silicon nitride (KemaNord P95-H) is employed containing 1.81percent oxygen, 0.6 percent carbon, and the following major metallicimpurities: 641 ppm iron, 315 ppm Al, and 25 ppm Ti. The silicon nitrideis present in the alpha and beta crystalline forms in an α/β weightratio of 95/5. The BET surface area of the silicon nitride powder is10.15 m² /g and the average particle size is about 1 μm in diameter.Magnesium oxide (J. T. Baker) is employed containing less than 5 ppmeach of boron, zinc, and iron. Greater than 80 percent of the MgOparticles range in size from 0.2 μm to 0.7 μm in diameter. Yttrium oxide(Molycorp) is employed containing less than 10 ppm each of sodium andiron. The Y₂ O₃ particles range in size from 2 μm to 5 μm in diameter.Calcium oxide (Aldrich Chemical Co.) is employed containing less than0.002 percent each of lead and iron. The average CaO particle size isabout 3 μm in diameter.

The above-identified magnesium oxide (4.7 g) and calcium oxide powders(0.2 g) are suspended in 80 ml of water, and agitated at roomtemperature under air by means of a mechanical stirrer to form atwo-oxide suspension. The pH of the suspension is adjusted to 11.35 bythe addition of aqueous sodium silicate (7 drops). After adjustment ofthe pH, the suspension is ultrasonicated for 30 seconds to break downfine agglomerates. After sonication the pH is observed to drop. The pHis readjusted to 11.5 by adding 5 drops of 5M sodium hydroxide. Thesuspension is mixed for about 30 minutes. Yttrium oxide powder (8.5 g),described hereinabove, is added to the suspension, and the suspension issonicated for 30 seconds and mixed with a mechanical stirrer for 30minutes. Silicon nitride powder (86.6 g), described hereinabove, isadded to the suspension, and the suspension is mixed in an attritionmixer for about 30 minutes to ensure complete dispersion of allcomponents. The resulting suspension is poured through a 100 mesh nylonsieve. The pH is adjusted to 9.8 by adding 10 ml of 50 percent nitricacid to increase the flocculation slightly. The finely dividedsuspension is dried in an oven at 90° C. for a period of 12 hours undera flow of dry nitrogen gas. After drying, the resulting powder mixtureis passed through a 60 mesh sieve. The powder mixture is composed of86.6 percent silicon nitride, 4.5 percent magnesium oxide, 8.5 percentyttrium oxide, and 0.2 percent calcium oxide.

The powder mixture (80 g), described hereinabove, is poured into agraphite die in the shape of plates measuring 2 inches in length by 1.5inches in width by 0.5 inches in depth. A pressure of 1000 psig isapplied to the die, while the temperature is raised from ambient toabout 1200° C. in about 30 minutes. At about 1200° C. the pressure isgradually increased to 5000 psig and maintained thereat. The temperatureis then increased to 1825° C. over a 40-minute period. The die ismaintained at 1825° C. and a pressure of 5000 psig for 45 minutes.Afterwards the die is cooled over a 2 hour period to 100° C. At 1500° C.the pressure is slowly released. When the die reaches room temperature,it is opened, and a silicon nitride ceramic body in the shape of a platehaving the above-identified dimensions is retrieved.

The density of the silicon nitride ceramic body, prepared hereinabove,is measured by the water immersion method, as described in "ModernCeramic Engineering" by D. W. Richerson, Marcel Dekker, 1982, and bystereology analysis from photomicrographs. The density is essentially100 percent of theoretical, and therefore the material is essentiallynonporous. Silicon nitride is present essentially in the β crystallinephase, as determined by X-ray diffraction. The bulk chemical compositionof the ceramic is determined by neutron activation analysis, and isfound to contain 77.2 percent silicon nitride, 20.4 percent glassysecond phase, and 2.4 percent silicon carbide. The glassy second phaseis found to consist of 32.4 percent magnesium oxide, 42.2 percentyttrium oxide, 2.5 percent calcium oxide, and 23.0 percent siliconoxide. Two unidentified phases are found. The first is present in aquantity of 3.9 percent, and possesses a composition of 9 percentmagnesium, 59 percent silicon and 32 percent nitrogen. The second ispresent in a quantity of 1 percent, and possesses a fiber-like, layeredand ordered structure typically 500 Å in width and 0.7 μm in length. Themicrostructure of the silicon nitride ceramic, prepared hereinabove, isdetermined by scanning electron microscopy (SEM), as viewed in a plane.About 35 volume percent of the silicon nitride appears in the form ofelongated whiskers having an aspect ratio ranging from 2 to 16. Theaverage aspect ratio is 5.6.

The fracture strength of the above-identified silicon nitride ceramicbody, measured by the 4-point bend test described hereinbefore, is 130ksi (890 MPa) at room temperature and 90 ksi (616 MPa) at 1000° C. Thefracture toughness measured by the Chevron notch technique is 13.9 MPa(m)^(1/2) at room temperature and 11.5 MPa (m)^(1/2) at 1000° C. TheVickers hardness measured at room temperature and under a 14-kg loadranges from 1350 kg/mm² to 1400 kg/mm² and averages 1375 kg/mm². ThePalmqvist toughness measured at room temperature ranges from 49.3 kg/mmto 51.1 kg/mm. It is seen that the fracture toughness of this siliconnitride ceramic body is very high.

EXAMPLES 2 (a-n)

A series of hot-pressed silicon nitride ceramic compositions is preparedaccording to the procedure of Example 1, except that the composition ofthe powder mixture is varied as described in Table I. The Vickershardness and the Palmqvist toughness measured at room temperature arepresented in Table I for each composition.

                                      TABLE I*                                    __________________________________________________________________________                                 Palmqvist                                                                           Vickers                                                                 Toughness                                                                           Hardness                                   Ex. 2                                                                            % Si.sub.3 N.sub.4                                                                 % MgO                                                                              % Y.sub.2 O.sub.3                                                                  % CaO                                                                              Y.sub.2 O.sub.3 /MgO                                                                (kg/mm)                                                                             (kg/mm.sup.2)                              __________________________________________________________________________    a  86.10                                                                              2.65 11.20                                                                              0.05 4.23  36.9  1461                                       b  86.45                                                                              3.50 10.00                                                                              0.05 2.85  40.2  1403                                       c  86.65                                                                              4.30 9.00 0.05 2.10  39.6  1353                                       d  86.75                                                                              4.70 8.50 0.05 1.81  42.2  1442                                       e  87.05                                                                              5.40 7.50 0.05 1.39  40.6  1392                                       f  87.25                                                                              6.10 6.60 0.05 1.10  42.4  1379                                       g  87.65                                                                              7.20 5.10 0.05 0.70  38.9  1324                                       h  86.40                                                                              3.50 10.00                                                                              0.10 2.85  44.5  1392                                       i  86.70                                                                              4.70 8.50 0.10 1.81  47.5  1401                                       j  87.60                                                                              7.20 5.10 0.10 0.70  40.2  1385                                       k  86.60                                                                              4.70 8.50 0.20 1.81  51.0  1391                                       l  86.60                                                                              4.70 8.20 0.50 1.82  47.6  1380                                       m  86.30                                                                              4.50 8.20 1.00 1.82  44.2  1375                                       n  85.30                                                                              4.50 8.20 2.00 1.82  40.1  1370                                       __________________________________________________________________________     *Percentages of components are based on weight percent in the powder          mixture. Toughness and hardness values are measured at room temperature. 

The data show that the Palmqvist toughness and the Vickers hardness varyas a function of the calcium oxide concentration and the Y₂ O₃ /MgOweight ratio in the powder mixture. For example, it is seen in Examples2(d,i,k,l,m,n) that as the calcium oxide concentration increases atconstant Y₂ O₃ /MgO ratio, the fracture toughness passes through amaximum value of 51 kg/mm at a calcium oxide concentration of 0.20weight percent. As the calcium oxide concentration increases at constantY₂ O₃ /MgO ratio, the hardness decreases.

COMPARATIVE EXPERIMENTS 1(a-d)

Four hot-pressed silicon nitride ceramic bodies are prepared as inExample 1, except that calcium oxide is omitted from the preparation.The powder compositions are listed in Table II. The Vickers hardness andthe Palmqvist toughness are measured as described in Example 2, and thevalues are tabulated in Table II.

                                      TABLE II*                                   __________________________________________________________________________                               Palmqvist                                                                           Hard-                                               %                   Toughness                                                                           ness                                         Comp. Ex. 1                                                                          Si.sub.3 N.sub.4                                                                  % MgO                                                                              % Y.sub.2 O.sub.3                                                                  Y.sub.2 O.sub.3 /MgO                                                                (kg/mm)                                                                             (kg/mm.sup.2)                                __________________________________________________________________________    a      86.40                                                                             3.60 10.00                                                                              2.78  36.6  1407                                         b      86.80                                                                             4.70 8.50 1.81  36.1  1377                                         c      87.75                                                                             7.20 5.05 0.70  36.1  1438                                         d      93.20                                                                             2.42 4.38 1.81  35.8  1395                                         __________________________________________________________________________     *Percentages of components are based on weight percent in the powder          mixture. Toughness and hardness values are measured at room temperature. 

When Comparative Experiments 1(b,d) are compared with Examples2(d,i,k,l,m,n) it is seen that the self-reinforced silicon nitrideceramic body of this invention possesses a significantly higherPalmqvist toughness than the samples which do not contain calcium oxide.The same conclusion holds on comparing Comparative Experiment 1(a) withExamples 2(b and h). Even at a low Y₂ O₃ /MgO ratio and a low calciumoxide concentration, the improvement in the ceramic body of thisinvention is noticeable, as seen in the comparison between ComparativeExperiment 1(c) and Examples 2(g and j).

EXAMPLES 3(a-c)

Three hot-pressed silicon nitride compositions are prepared according tothe procedure of Example 1, except that the powder compositions arevaried as described in Table III. The Y₂ O₃ /MgO ratio in these powdercompositions is 1.82. The Palmqvist toughness and the Vickers hardnessare measured at room temperature according to the procedure in Example2. The values obtained are presented in Table III.

                                      TABLE III*                                  __________________________________________________________________________                               Palmqvist                                                                           Hard-                                           %                  % Glass                                                                            Toughness                                                                           ness                                         Ex. 3                                                                            Si.sub.3 N.sub.4                                                                  % MgO                                                                              % Y.sub.2 O.sub.3                                                                  % CaO                                                                              Content                                                                            (kg/mm)                                                                             (kg/mm.sup.2)                                __________________________________________________________________________    a  93.15                                                                             2.40 4.35 0.10 6.85 39.9  1371                                         b  86.70                                                                             4.70 8.50 0.10 13.30                                                                              46.2  1396                                         c  80.40                                                                             6.90 12.60                                                                              0.10 19.60                                                                              48.3  1389                                         __________________________________________________________________________     *Percentages of components are based on weight percent in the powder.         Toughness and hardness values are measured at room temperature.          

The data show that as the glass content increases, the Palmqvisttoughness also increases; whereas the Vickers hardness varies in anon-linear fashion.

EXAMPLE 4

The hot-pressed silicon nitride ceramic body of Example 1 is diamondground into a cutting tool insert. The cutting tool insert is madeaccording to the ANSI standards in the SNG 433 style. The cutting edgeis chamfered at a 20° angle by 0.008-inch width. The insert is tested ina face milling application using a 40 HP Cincinnati #5 single spindle,knee and saddle, vertical milling machine with a 5 HP variable speedtable. The work material is a nodular "ductile" cast iron measuring 2inches in diameter and having a measured hardness of 207 BHN. A millingcutter having a 6-inch diameter is used with a -5° axial rake and a -5°radial rake. A 15° lead angle is employed. The machine is run at acutting speed of 1360 surface feet per minute, a 0.060-inch depth ofcut, and a feed rate of 0.005 inch per revolution (or tooth). The centerline of the cutter and the center line of the workpiece are coincident.No cutting fluid is used. Successive passes are taken on the cast iron,and the cutting edge is examined for flank wear and chippage after every8 passes. Testing is terminated when the flank wear or chippage exceeds0.015 inch in depth as measured with a 30-power microscope. It is foundthat an average of 26.0 passes are achieved prior to failure. The flankwear is uniform.

COMPARATIVE EXPERIMENT 2 (a-b)

A commercial silicon nitride ceramic body is obtained from each of thefollowing sources: (a) Boride Products (Product No. US-20) and (b) GTEValeron Corporation (Product No. Q6). Each sample is diamond ground intoa cutting tool in the manner described in Example 4. The cutting toolsare used to cut nodular "ductile" cast iron in the manner described inExample 4. It is found that an average of 13.5 passes of the BorideProducts sample are achieved prior to failure, and an average of 10.5passes of the GTE sample are achieved prior to failure. In both casesthe flank wear is uniform. When Comparative Experiments 2(a and b) arecompared with Example 4, it is seen that the silicon nitride ceramicbody of this invention significantly outperforms the commercialproducts.

What is claimed is:
 1. A silicon nitride ceramic body having a fracturetoughness greater than about 6 MPa (m)^(1/2), as measured by the Chevronnotch technique at about 23° C., consisting essentially of(a) acrystalline phase of β-silicon nitride of which at least about 20 volumepercent, as measured in a plane by scanning electron microscopy, is inthe form of whiskers having an average aspect ratio of at least about2.5; and (b) a glassy phase in an amount not greater than about 35weight percent of the total weight comprising magnesium oxide in a rangefrom about 15 percent to about 51 percent, yttrium oxide in a range fromabout 36 percent to about 63 percent, calcium oxide in a range fromabout 0.1 percent to about 15 percent, and silica in a range from about3 percent to about 30 percent by weight.
 2. The composition of claim 1wherein the weight ratio of yttrium oxide to magnesium oxide is in arange from about 0.7 to about 4.2.; and wherein not greater than about10 weight percent of the total weight is present as other phases.
 3. Thecomposition of claim 1 wherein the percentage of silicon nitridewhiskers is at least about 35 volume percent.
 4. The composition ofclaim 1 wherein the whiskers have an average aspect ratio of at leastabout 5.5.
 5. The composition of claim 1 wherein the fracture toughnessis greater than about 7 MPa (m)^(1/2).
 6. The composition of claim 5wherein the fracture toughness ranges from about 9 MPa (m)^(1/2) toabout 14 MPa (m)^(1/2).
 7. The composition of claim 1 wherein thefracture toughness as measured by the Chevron notch technique at 1000°C. ranges from about 7 MPa (m)^(1/2) to about 12 MPa (m)^(1/2).
 8. Thecomposition of claim 1 wherein the Palmqvist toughness measured at about23° C. ranges from about 37 kg/mm to about 52 kg/mm.
 9. The compositionof claim 8 wherein the Palmqvist toughness measured at about 23° C.ranges from about 45 kg/mm to about 52 kg/mm.
 10. The composition ofclaim 1 wherein the fracture strength measured at room temperature is inthe range from about 685 MPa to about 925 MPa.
 11. The composition ofclaim 1 wherein the fracture strength measured at 1000° C. is in therange from about 580 MPa to about 650 MPa.
 12. A cutting tool fabricatedfrom the composition of claim 1.